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EP-2016-092

(Submitted on 03 May 2016)

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Abstract

The production of $W$ and $Z$ bosons in association with jets is studied in
the forward region of proton-proton collisions collected at a centre-of-mass
energy of 8 TeV by the LHCb experiment, corresponding to an integrated
luminosity of 1.98 $\pm$ 0.02 fb$^{-1}$. The $W$ boson is identified using its
decay to a muon and a neutrino, while the $Z$ boson is identified through its
decay to a muon pair. Total cross-sections are measured and combined into
charge ratios, asymmetries, and ratios of $W+$jet and $Z$+jet production
cross-sections. Differential measurements are also performed as a function of
both boson and jet kinematic variables. All results are in agreement with
Standard Model predictions.

Figures and captions

The contributions to the selected (left) $ W ^+ {j}$ and (right) $ W ^- {j}$ samples are shown, where the QCD background is obtained by a fit to the $ p_{\textrm{T}}^{\mu}$ / $ p_{\textrm{T}}^{\mu\textrm{-jet} }$ spectrum and the electroweak background is determined as described in the text. The contributions shown are the sum of the individual contributions in bins of $\eta^{\textrm{jet}}$ , where the charge asymmetry typical of $ W {j}$ production in $pp$ collisions is evident.

Summary of the measurements performed in the fiducial region, as defined in Sec. 1. The measurements are shown as bands, while the theoretical predictions are presented as points. For the experimental measurements, the inner band represents the statistical uncertainty, while the outer band represents the total uncertainty. For the theory points, the inner error bar represents the scale uncertainty, while the outer bar represents the total uncertainty. The cross-sections and ratios are shown normalised to the measurement, while the asymmetry is presented separately.

$ W {j}$ bin-averaged differential cross-sections as a function of $\eta^{\mu}$ (left) and $\eta^{\textrm{jet}}$ (right). The measurements are shown as bands representing the statistical and total uncertainties, while the theoretical predictions are shown as points (displaced horizontally for presentation) representing the same bin-averaged cross-sections as the data. The inner error bar represents the scale uncertainty, and the outer error bar represents the total uncertainty. The ratio of the predicted to measured cross-sections is shown below the distribution. The $ W ^+ {j}$ and $ W ^- {j}$ cross-sections are seen to overlap in the final bin in $\eta^{\mu}$ .

$ W {j}$ bin-averaged differential cross-sections as a function of $ p_{\textrm{T}}^{\textrm{jet}}$ . The experimental and theoretical components are shown as in Fig. 3. The ratio of the predicted to measured cross-sections is shown below the distribution.

The measured bin-averaged differential $ Z {j}$ production cross-section is shown as a function of (left) $\eta^{\textrm{jet}}$ and (right) $ p_{\textrm{T}}^{\textrm{jet}}$ . The experimental measurements are shown as bands, while the theoretical predictions are shown as points, horizontally displaced for presentation. The ratio of the predicted to measured cross-sections is shown below the distribution.

The measured bin-averaged differential $ Z {j}$ production cross-section is shown as a function of (left) $ y^{ Z }$ and (right) azimuthal separation between the $ Z $ boson and the jet. The experimental measurements are shown as bands, while the theoretical predictions are shown as points, horizontally displaced for presentation. The ratio of the predicted to measured cross-sections is shown below the distribution.

Ratio (left) and asymmetry (right) of $ W ^+ {j}$ to $ W ^- {j}$ production as a function of the lepton pseudorapidity. The experimental measurements are shown as bands, while the theoretical predictions are shown as points, horizontally displaced for presentation. The ratio of the predictions to the experimentally measured values is shown below the distribution for the charge ratio, while their difference is shown for the charge asymmetry.

Ratio (left) and asymmetry (right) of $ W ^+ {j}$ to $ W ^- {j}$ production as a function of the lepton pseudorapidity compared to NLO calculations performed with the Fewz generator and three different PDF sets. The experimental measurements are shown as bands, while the theoretical predictions are shown as points, horizontally displaced for presentation. The same comparisons are shown below the distribution as described in Fig. 7.